The photoelectric factor (P.sub.e) is commonly employed to enhance lithology determination in a borehole environment. It is defined as the formation effective photoelectric absorption cross section and represents an approximation of the photoelectric absorption properties of the formation, which may vary widely from element to element. When combined with formation porosity and bulk density measurements to obtain a volumetric matrix photoelectric cross section, P.sub.e can thus be used to identify formation lithology. The measurement of P.sub.e in boreholes is made with density/lithology tools by combining count rates of high-energy and low-energy gamma rays. The traditional reasoning in correlating P.sub.e to these count rates is based on the assumption that Compton scattering and photoelectric absorption are the only significant interaction mechanisms. Unfortunately, this assumption does not consider Rayleigh scattering and binding-energy corrections to Compton scattering, which are significant for the low-energy gammas used in the P.sub.e measurement and prevent the tools from measuring P.sub.e exactly. Although these effects are minimized by calibrating the tool to P.sub.e values in pure limestone and sandstone, errors persist for other lithologies and when induced by the presence of high-atomic number elements may be significant. Although these errors are typically less than 4 percent in measurements made by a short-spaced detector with an approximate 5 inch spacing between source and detector, they can be as large as 8 percent in halite and 280 percent in coal. In a long-spaced tool measurement, with approximate 12 inch spacing between source and detector, the errors are only slightly smaller.
The presence of Rayleigh scattering and binding-energy corrections to Compton scattering in all low-energy scattering processes also complicates the laboratory measurement of P.sub.e, which measurements are useful in determining accurate values for P.sub.e standards and in comparing tool response to core samples from wells. Previously, to check or calibrate a tool, the P.sub.e of samples has been determined by calibrations from an elemental analysis performed on a core sample. However, there are ddrawbacks to this method in that the calculation of P.sub.e values accurate to 0.05 can be difficult to achieve, due to the presence of elements that were omitted in the analysis or to uncertainties in the analyses of high-atomic-number elements. Another drawback is that the equation used to calculate P.sub.e only approximates photoelectric absorption, and this approximation introduces large errors when high-atomic number elements are present, e.g. barium at the 0.1 percent level.
To reduce errors caused by the high-atomic member elements, others have made P.sub.e measurements using a bremsstrahlung x-ray beam in a method which requires an elemental analysis of the sample to extract Rayleigh scattering effects. Such method is predicated on the thesis that it is better to use the elemental analysis to calculate a correction to the P.sub.e measurement than to use it to calculate the P.sub.e factor itself, and although the method removes some of the inaccuracies of P.sub.e determination, it has not removed the need for an elemental analysis.